Office equipment such as photocopiers, laser printers, ink jet printers and other imprinting mechanisms today incorporate an ability to receive sheet media from any one of two or more trays. It is very common to have a bypass tray in which a single sheet medium of a special type or of a different size can be conveniently fed for particular printing without having to load it onto pull-out trays. Such a bypass tray is useful for tasks such as printing a formal document on a sheet medium with a pre-printed company letterhead, printing on a single sheet of transparency and printing a final copy of a document on a better quality sheet medium. In these circumstances, it will be convenient for a user to simply load the appropriate sheet medium to be printed on an exposed bypass tray. Such bypass trays are commonly found in small-format printers, for example the ink jet printers for printing on smaller width (A4 and Letter size) sheet media. These printers are popular amongst home and general office users. However, with advancement in speed and resolution of such printers, these ink jet printers have now become appealing to a different segment of users which requires printing on larger width sheet media, such as B4 size paper.
One prior art media handling system has a pressure plate pivotably mounted on a base of a printer. During a pick cycle, the pressure plate is lifted to move a stack of sheet media located above it to an infeed zone of the printer. This pressure plate is usually designed to directly support a stack of sheet media in a main tray. During each pick cycle, the sheet at the top of the stack will be moved to the infeed zone. A simple and economical design of a bypass feeding system usually has a bypass tray positioned above the main tray. Sheet media in this bypass tray is supported either by the pressure plate itself (when there is no sheet medium in the main tray) or by a stack of sheet media in the main tray. Such a design accords pick priority to any sheet medium in the bypass tray over that in the main tray. This design works well when the sheet media on both the bypass and main trays are of substantially the same width. When the media are of substantially the same widths, the sheet media in the main tray will be able to provide sufficient support for the sheet media in the bypass tray.
However, a serious problem results when the sheet medium in the bypass tray is substantially wider than a stack of sheet media in the main tray. For example, the sheet media in the bypass tray is of B4 size and the sheet media in the main tray is of A4 or Letter size. A substantially large portion of the B4 size sheet medium in the bypass tray will not be supported by the A4 or Letter size sheet media in the main tray. The problem is not so serious when there is only a small stack of sheet media in the main tray. A small stack will create only a small height differential between portions of the B4 size sheet medium. In such a case, the B4 size sheet medium is still fairly well supported, partly by the stack of sheet media in the main tray and partly by the exposed portion of the pressure plate. As a result, the B4 size sheet medium can be properly presented for picking by the pick mechanism in the printer. However, when the stack height of the smaller width sheet media in the main tray measures half an inch or more, a relatively large portion of the B4 size sheet medium would not be supported by the pressure plate and would sag at the arris of the stack of sheet media in the main tray. Such sagging is detrimental to the aligned feeding of the B4 size sheet medium into the printer as the sheet medium will not be properly presented to the pick mechanism. The pick mechanism is only able to properly engage the portion of the B4 size sheet medium that is supported by the stack of sheet media in the main tray. As a result, the properly engaged portion will be drawn into the printer ahead of the rest of the sheet medium. Such an action would cause the sheet medium to follow an oblique course or a deviation from a predetermined straight line path when being received into the printer. Such unbalanced drawing of the B4 size sheet medium results in skewing of the sheet medium. This skewing causes undesirable result, for text printed on the skewed sheet medium will appear misaligned, crooked or oblique. In some cases, when the skew is severe, the sheet medium may end up jammed in the printer.
To prevent the unbalanced support or sagging of the B4 size sheet medium, a user can remove the stack of smaller width sheet media from the main tray before printing on the B4 size sheet medium. The removal of the sheet media in the main tray will result in the B4 size sheet medium being fully supported by the pressure plate. However, such extraneous removal of the sheet media from the main tray defeats the purpose of having a bypass tray. It is cumbersome and unacceptable from the point of usability for a user to have to empty and replenish the main tray each time a larger width sheet medium is to be printed.
From the foregoing, the prior art therefore has a need for an improved sheet media handling system which is able to provide aligned feeding of a larger width sheet medium when placed over a stack of smaller width sheet media.